The present invention relates to an improved laser-assisted intrastromal in-situ keratomileusis (xe2x80x9cLASIKxe2x80x9d) procedure. More particularly, it relates to a LASIK procedure in which intraocular pressure is measured prior to cutting a flap in the cornea, along with a tonometer system capable of providing elevated intraocular pressure readings.
LASIK procedures have recently become highly popular for correcting various vision problems such as myopia, astigmatism, and hyperopia. In general terms, an automated microkeratome is employed to cut a large, superficial flap in the cornea. The flap is typically 160-180 microns in depth so as to include the first two corneal layers (i.e., the epithelium and Bowmans layer), while leaving the third layer or stroma in tact. The arc length of the so-defined flap is typically on the order of 300xc2x0, such that the flap can be folded back to expose the stroma. An excimer laser is then used to ablate or reshape the corneal stroma to effectuate the desired vision correction or enhancement. To this end, the excimer laser is typically centered on the prepupillary area. Following ablation, the flap is replaced or folded back. After a few minutes, the flap remains in place without requiring sutures, much like a suction cup.
As with other popular surgical procedures, numerous efforts have been made to improve the microkeratome and excimer laser instruments. One distinct aspect of the LASIK procedure, however, remains relatively rudimentary. In particular, to properly form the desired flap, the eyeball is subjected to a vacuum prior to cutting with the microkeratome. To this end, the microkeratome device includes a suction ring placed about the eye for applying the vacuum, resulting in a greatly elevated intraocular pressure as well as a slight distension of the eyeball from the socket. The elevated intraocular pressure renders the eyeball rigid, thereby promoting uniform, properly-sized incision by the microkeratome. Conversely, inadequate intraocular pressure can undesirably result in a thin, irregular flap.
As a point of reference, normal intraocular pressure is approximately 15-20 mm Hg. During a LASIK procedure, however, the intraocular pressure is preferably raised to a level in excess of 80 mm Hg to ensure proper eyeball rigidity. The optimal intraocular pressure for uniform microkeratome incision is currently unknown, due to the fact that available intraocular pressure sensing devices (or tonometers) have an upper limit at or below 80 mm Hg, as described in greater detail below. Thus, the currently accepted technique of evaluating intraocular pressure prior to microkeratome flap formation is known as a xe2x80x9cfingerxe2x80x9d or xe2x80x9cdigitxe2x80x9d method, whereby the surgeon touches the patient""s eye with his/her finger and speculates as to the intraocular pressure. Obviously, evaluating intraocular pressure with the finger is highly subjective as the surgeon is required to guess as to whether the eye xe2x80x9cfeelsxe2x80x9d sufficiently rigid.
As indicated above, various tonometry instruments do exist for measuring intraocular pressure. Generally, there are two types of tonometers, including applanation and indentation. Either approach requires numbing of the eye and entails applying a force, via a probe placed against the cornea, that produces a distortion of the eyeball globe. The pressure required to applanate or indent the cornea is indicative of the intraocular pressure.
The most popular form of indentation tonometer is known as a Schiotz tonometer. With the Schiotz tonometer, a plunger produces a corneal indentation, the depth and volume of which are dependent upon the intraocular pressure. Other indentation tonometers, such as a Wolfe tonometer, rely on similar principals but with different internal or probe components, such as a calibrated spring.
An applanation tonometer measures the force required to flatten a certain area of the cornea. Examples of applanation tonometers include Goldmann tonometer, Draeger applanation tonometer, Mackay-Marg tonometer, and pneumatonometer, to name but a few. The Goldmann tonometer was the first example of a variable force applanation tonometer, and is still considered to be highly reliable. Other devices, such as the Mackay-Marg tonometer, measure intraocular pressure by analyzing deflections in membranes or crystals when the device is pressed against the eye. A pneumatonometer probe contains a gas and employs a pressure sensor or transducer. The pneumatonometer determines intraocular pressure by bringing a small burst of air toward the cornea. A backpressure is sensed, and is proportional to the intraocular pressure.
A third tonometry technique is generally referred to as xe2x80x9cnon-contactxe2x80x9d and uses a stream of air (or xe2x80x9cpuffxe2x80x9d) to flatten a portion of the cornea, while a light source is reflected off of that flattened portion. The pressure or force required to cause the sensed displacement is indicative of the intraocular pressure. While non-contact tonometers do not require anesthetizing the eye prior to use, they are generally regarded as being less accurate than applanation or indentation tonometers.
Each of the above-described tonometer systems were developed to detect the onset of glaucoma. Glaucoma is typically characterized by an increased intraocular pressure, above the normal 20 mm Hg. In this regard, a patient exhibiting a sensed intraocular pressure in excess of 30 mm Hg is typically deemed to be at risk for glaucoma; a 40-50 mm Hg intraocular pressure is highly abnormal, resulting in immediate emergency treatment. With these general parameters in mind, then, virtually all available tonometer devices have a maximum reading of approximately 50 mm Hg. In other words, for glaucoma testing, there is no need to provide for a larger range. Further, by limiting the range to approximately 50 mm Hg, the overall accuracy of the device can be increased.
At least one currently available pneumatonometer is able to accurately measure pressure up to 80 mm Hg. In particular, the Model 30 Classic(trademark) pneumatonometer, available from Medtronic-Solan (formerly Mentor Ophthalmics) of Jacksonville, Fla., provides an intraocular pressure measurement in the range of 5-80 mm Hg. The Model 30 Classic(trademark) device provides this enhanced intraocular pressure range to perform not only tonometer functions, but also tonography (i.e., evaluation of aqueous outflow of the eye). The accuracy of the Model 30 Classic(trademark) is +/xe2x88x922 mm Hg, and therefore is highly satisfactory for tonometery applications. It is believed that one other available tonometer has an upper limit of 90 mm Hg, but is limited to a simple xe2x80x9cgo or no-goxe2x80x9d indication for intraocular pressure in excess of 50 mm Hg. Beyond these two examples, no device exists for measuring intraocular pressures in excess of 50 mm Hg.
The microkeratome and excimer laser instruments used for LASIK procedures continue to improve. However, the xe2x80x9cfinger touchxe2x80x9d technique of evaluating intraocular pressure prior to microkeratome incision presents a distinct opportunity for complications. Therefore, a need exists for an improved LASIK procedure that promotes optimal flap formation and a tonometry system for effectuating this procedure.
One aspect of the present invention relates to a method of performing a LASIK procedure. The method includes providing LASIK instrumentation, preferably including an automated microkeratome instrument and an excimer laser. The microkeratome device includes both a microkeratome as well as a suction ring. A high-pressure range tonometer instrument is also provided. The tonometer includes a probe for sensing a parameter indicative of intraocular pressure, and is capable of providing intraocular pressure measurements in excess of 80 mm Hg; more preferably in excess of 100 mm Hg; most preferably up to or in excess of 120 mm Hg.
With these components in hand, the microkeratome instrument is positioned relative to the patient, including applying the suction ring over the patient""s eye. A vacuum pressure is then applied to the eye via the suction ring. The high-pressure range tonometer probe is positioned relative to the eye so as to sense a parameter indicative of intraocular pressure. The measured intraocular pressure is then provided to the surgeon. Based upon the measured intraocular pressure, the surgeon determines whether the corneal site is properly prepared for optimal microkeratome incision. For example, the surgeon determines whether the measured intraocular pressure meets or exceeds a minimum value. In one preferred embodiment, the intraocular pressure must be equal to or exceed 100 mm Hg, although other minimum values, such as 110 mm Hg, 120 mm Hg, etc. are equally acceptable. Once sufficient intraocular pressure has been established, the LASIK procedure proceeds in accordance with accepted protocol. For example, the microkeratome is employed to cut a flap in the cornea that is then folded back. The excimer laser is used to ablate the exposed portion of the cornea to effectuate the desired vision correction. Finally, the flap is replaced and the surgery is complete. In one preferred embodiment, an existing pneumatonometer is reconfigured to provide intraocular pressure measurements up to, or in excess of, 120 mm Hg for use with the above-described method.
Another aspect of the present invention relates to a method of measuring an intraocular pressure in excess of 80 mm Hg as part of LASIK procedure. The method includes providing a tonometer having a maximum intraocular pressure measuring capability of less than 80 mm Hg. A vacuum pressure is then applied to a patient""s eye. A range setting is established for the tonometer. Following application of the vacuum, an initial intraocular pressure measurement for the eye is received from the tonometer. A corrected intraocular pressure measurement is then determined based upon the initial intraocular pressure measurement in the range setting. Finally, the corrected intraocular pressure measurement is displayed. With this methodology, a corrected intraocular pressure measurement of greater than 80 mm Hg can be measured and displayed. In one preferred embodiment, the range setting of the tonometer is established as being either xe2x80x9clowxe2x80x9d or xe2x80x9chighxe2x80x9d. In an even more preferred embodiment, when the tonometer is designated as operating in the high range, a factor of 40 mm Hg is added to the initial intraocular pressure measurement.
Yet another aspect of the present invention relates to a system for measuring an intraocular pressure in excess of 80 mm Hg for patient""s eye undergoing an LASIK procedure. The system includes a tonometer and a processor. The tonometer is capable of measuring a maximum intraocular pressure of less than 80 mm Hg. The processor is associated with the tonometer for receiving intraocular pressure measurements from the tonometer. Further, the processor is capable of establishing a range setting for the tonometer. The processor is further capable of receiving an initial intraocular pressure measurement from the tonometer and determining a corrected intraocular pressure measurement based upon the initial intraocular measurement and the range setting. Finally, the processor is capable of displaying the corrected intraocular pressure measurement. With the system of the present invention, an intraocular pressure of greater than 80 mm Hg can be provided via a tonometer having limited pressure applications. Thus, the system is highly useful with LASIK procedures.